Photonic crystals are spatially periodic structures having useful electromagnetic wave properties, such as photonic bandgaps. In principle, the spatial periodicity of a photonic crystal can be in one, two, or three dimensions. There is especially high interest in developing technology of artificial photonic crystals that are useful in new and improved functional photonic devices, especially for the infrared and visible-light portions of the electromagnetic spectrum. Functional devices using photonic crystals, such as selective reflectors, filters, optical couplers, resonant cavities, delay lines, and waveguides have been proposed and/or fabricated.
Several methods for forming artificial photonic crystals are known. Multilayered dielectric films have been used to make one-dimensional photonic crystals along the dimension perpendicular to the films.
Three-dimensional photonic crystals have been formed by stacking and bonding wafers in which periodic structures have been micromachined by etching. Such methods result in structures called “wood-pile” or “picket-fence” structures because the stacked elements have an appearance similar to stacked square timbers. Such methods require precise alignment of the micromachined wafers to be bonded together, which becomes more difficult as the number of layers increases and as the dimensions of micromachined features are reduced.
Some of the known methods for forming artificial photonic crystals work by modifying refractive index periodically in a material originally having a uniform refractive index. For example, light-wave interference or holography has been used to create periodic variations of refractive index within photosensitive materials, such as photoresist, to make photonic crystals. Perhaps the simplest methods for forming a one- or two-dimensional photonic crystal are those methods that form a periodic or quasi-periodic array of holes in a uniform slab of material. A vacuum or material filling the holes has a different index of refraction from the base material of the slab. In the background art, such holes have been formed by micro-machining or by nanoscale lithography, such as electron-beam or ion-beam lithography. Conversely, such charged-particle beam lithography has also been used to selectively assist deposition of material to form spaced elements of the photonic crystal. Some photonic crystals have been formed by self-assembly of very small particles provided in a colloidal suspension. In some cases, the interstitial spaces between the colloidal particles have been filled with a second material of a different refractive index. In some of those cases, the colloidal particles themselves have been removed to leave an “inverse” photonic crystal in which the crystal lattice positions are occupied by voids in a matrix of the second material.
Photonic-crystal filaments have been proposed to exploit the bandgap properties of photonic crystals for improving the luminous efficiency of lamp filaments, reducing the amount of infrared emission in favor of increased emission of visible-light wavelengths. Prototype photonic crystals for such purposes have been fabricated by extensions of known Micro Electro-Mechanical Systems (MEMS) technology. Technology for making suitable metallic photonic crystals is disclosed, for example, in the article by J. G. Fleming et al., “All-metallic three-dimensional photonic crystals with a large infrared bandgap,” Nature V. 417, No. 6884 (May 2, 2002), pp. 52-54.
While all of these methods and others have been used successfully to make photonic crystals in various forms, the need for efficient methods of mass-production and the particular requirements associated with filamentary photonic crystals, especially for use in lighting applications, create a need for fabrication methods specially adapted for photonic-crystal filaments.